Gold nanoparticles functionalized with oligonucleotides are important reagents in many highly sensitive and selective oligonucleotide and protein detection schemes. They are also key building blocks in several assembly schemes that utilize the chemically programmable, sequence specific hybridization properties of nucleic acids. The use of gold particles in biodetection stems from the existence of well established and straightforward methods for the functionalization of a gold surface with ligands containing thiol or disulfide groups, the cooperative binding properties of the probes, their intense optical properties, and their catalytic properties. Each of these properties leads to a selectivity or sensitivity advantage over conventional probes.
Magnetic particles also have been widely used in diagnostic and therapeutic applications. Although methods of synthesizing magnetic particles with control over size and shape exist, surface functionalization of magnetic particles with biomolecules often requires elaborate synthetic schemes.
There exists a need for synthetic methods that provide composite materials having both the stability, surface chemistry, and optical properties of gold particles and the magnetic properties of superparamagnetic particles. Core-shell approaches to realizing such probe structures have met with limited success. Direct coating of magnetic particles with gold is a difficult task due to the dissimilar nature of the two surfaces. Methods based on the synthesis of one of the compositions in the presence of the other have led to structures with minimal interface contact between gold and the magnetic particles.